As regards small diameter tubes, the heat exchangers such as steam generators, in particular steam generators used in power-generating nuclear power stations, generally comprise a bundle of long length tubes which may be straight, bent or even helically coiled.
In particular, in the case of nuclear reactors cooled by pressurized water or by liquid sodium, the bundle of tubes is disposed inside an external cylindrical enclosure having a vertical axis, and the tubes are fixed, by their terminal portions, either to tube plates integral with the enclosure or to tube plates or manifolds exterior to the enclosure of the steam generator. In the latter case, each tube comprises an intermediate fastening, via a thermal sleeve, to the enclosure.
The fluid cooling the reactor flows either inside or outside the tubes of the bundle so as to heat and vaporize feedwater through their wall.
The walls of the tubes which constitute the heat exchange tubes therefore separate a fluid cooling the nuclear reactor from the feedwater to be vaporized.
In the case of nuclear reactors cooled by pressurized water, the fluid cooling the reactor, i.e., the primary coolant, comes into contact, inside the reactor vessel, with the core of the reactor constituted by the fuel assemblies, and is therefore liable to contain radioactive products. It is therefore essential to work under conditions such that, during manufacture and use of the steam generators, any leakage of the fluid cooling the reactor towards the feedwater which, after conversion to steam goes to the turbine, is avoided.
Likewise, in the case of fast neutron nuclear reactors cooled by a reactive metal such as liquid sodium, it is necessary to avoid any leakage through the wall of the exchange tubes being manifested by water or water vapor coming into contact with the liquid sodium and by an extremely vigorous reaction which can lead to explosions and to damage, at least in part, of the steam generator.
It is therefore necessary to carry out meticulous inspections of the wall of the heat exchange tubes at different stages of the manufacture of the steam generators, and after a certain time in use of these steam generators, in order to ensure complete integrity of the wall of these tubes separating the heat exchange fluids.
As regards larger diameter tubes, the device may be applied to the inspections of the walls and of the welds of the tubes conveying fluids such as hydrocarbons (pipeline), gases or any other fluids.
In general, it may be necessary to inspect the state of the wall of tubes during their manufacture, before or after their mounting and during periodic inspections, for example after a certain operation time of apparatuses or systems for which these tubes are used. It may also be necessary to inspect the state of the tubular walls of long length lines such as pipelines.
The tubes used in the apparatuses, or in the systems connecting apparatuses, or in the fluid-flow lines, may experience elevated temperatures and possibly high pressures.
As regards solid or hollow structural sections, it may be necessary to carry out the inspection of solid bars of circular or prismatic cross-section, or more especially of the rails for moving machinery or any element of elongate shape having a substantially constant transverse cross-section.
The purpose of the non-destructive inspections carried out on the tubes and structural sections may be to check that their integrity conforms to that required in construction or to reveal defects appearing in operation.
In order to carry out these inspections, it has been proposed to use various methods employing, for example, eddy current probes or radiographic examination techniques.
The devices used in employing these methods are generally bulky, so that it may be difficult, if not impossible, to cause them to pass at least into certain portions of the tubes, for example into the bent portions having a small bend radius. These techniques may also be completely inapplicable in the case of small diameter heat exchanger tubes.
In the case of steam generators associated with nuclear reactors, the exchange tubes may have an internal diameter less than 20 mm and a length of the order of 50 to 100 m.
Furthermore, eddy current methods have a reduced sensitivity in the detection of the defects of the welds of the tubes and in the inspection of ferromagnetic materials.
The deposits which may be present inside or outside the tubes, deposits which may be metallic and, for example, constituted by solidified sodium in the case of fast neutron steam generators and heat exchangers, are liable to interfere with the signal corresponding to the eddy currents and to decrease strongly the sensitivity of the detection.
The use of non-destructive inspection methods employing ultrasound has enabled some of the abovementioned drawbacks to be avoided, in the case of the inspection of small diameter tubes.
It has been proposed to use ultrasonic transducers emitting an ultrasonic beam in the direction of the wall of the tube to be inspected, i.e., in a radial plane. It is possible to associate with these transducers, mechanisms enabling the ultrasonic beam to be made to scan mechanically the tube in the circumferential direction or along a helix.
In order to carry out a non-destructive inspection of the wall of a tube, via the inside or via the outside of the tube, by circumferential or helical scanning, it has been proposed to use a device comprising an ultrasonic probe or transducer which may be translationally moved along the axial direction of the tube and which may be rotated about an axis coincident with the axis of the tube during the inspection.
The ultrasonic beam produced by the transducer may be emitted directly in the direction of the wall of the tube or reflected towards the wall by a mirror.
Such devices have the drawback of requiring the use of complex mechanisms such as fractional horsepower motors and speed-reducing gears enabling the transducer, and, possibly the mirror, to be rotatably driven, mechanical linkage means comprising small-size universal joints or even rotating commutators.
The mechanisms enabling the transducer and the motor to rotate are tricky to operate and may be sensitive to the presence of solid particles detached from the wall of the tube and which may be in suspension in the coupling fluid.
Because of the design of the transducer, the angle of incidence of the ultrasonic beam and the focal distance of this beam are fixed unless it is intended to use a mirror of complex shape or which is moved in a specially adapted manner.
Finally, such devices have a significant bulkiness in the longitudinal and/or diametral direction of the tube.
There are also known devices for ultrasonic inspection of small diameter tubes comprising a non-rotating transducer constituted by several piezoelectric elements placed in a single plane, juxtaposed or separated by insulation parts, so as to constitute an array which is symmetrical in revolution and in which each of the piezoelectric elements is capable of emitting an ultrasonic beam forming separate impact zones in the plane. Each beam makes it possible to analyze a specified impact zone of the tube, and the array of the beams emanating from the piezoelectric elements of a single plane does not therefore permit a complete analysis of the circumference of the tube.
The circumferential analysis of the wall of the tube by these points of impact is obtained by excitation, in a successive manner and in a given order, of each of the piezoelectric elements. However, taking into account the bulkiness of the elements, the points of impact of the beams constituting focal spots on the wall of the tube are relatively far apart in the emission plane of the transducer.
To increase the number of these points of impact, piezoelectric elements disposed along several rows and in staggered positions are used.
However, in this case, the drawbacks of the conventional-type transducers are encountered again. In particular, the angle of incidence of the beam and the focal distance of the ultrasonic waves are not modifiable. The device may be bulky in the longitudinal direction because it is necessary to use several rows of transducers spaced in the axial direction. Furthermore, while using the transducer, it is necessary to prevent any radial movement during its movement in the axial direction of the tube, so as not to lose the angular reference of the points examined.
In fact, the devices according to the prior art are generally bulky and cannot be used over the entire length of small diameter tubes having a significant length which can reach 100 mm and which are bent with small bend radii.
U.S. Pat. No. 3,693,415 discloses a scanning ultrasonic inspection method and device for the detection of cracks in a part such as a tube, in which transducers are disposed along a row around the part and are supplied by successive groups, so as to focus an ultrasonic beam onto successive points of the part during inspection. This method makes it possible to obtain a narrow focal spot in the circumferential direction of a tube during examination. However, in the axial direction of the tube, along which the transducers have a certain length, the focal spot extends over a length greater than the length of the transducers because of the divergence of the ultrasonic beam. This focal spot therefore has the shape of a "thick line". The resolution of the inspection device is therefore limited and it is not possible to detect cracks of small size.